“What was the MAIN reason for BRexit to begin with?”
There were always a variety of irritants (the UK is always a net Bill Payer into the EU kitty), but I believe the big one, and the one that broke the camel’s back, was the EU forcing them accept a lot of immigrants, who became public charges and security problems.
The EU didn't "force them to accept a lot of immigrants" As per the EU rules - to which the UK signed up to, all citizens of EU countries can freely travel and work (note: there are rules about the latter - I'll come to it). After 2004 nearly 2 million central europeans moved to the UK - but that's because the UK government didn't implement the EU regulations that allow EU countries to have put up blocks for 4 years on new country entrants -- Germany did this, the UK government didn't. The net result was that the UK grew faster than Germany from 2004 to 2012. The UK got an opt-out from the Schengen zone, so non-EU citizens who were residents of other EU countries could NOT travel to the UK (leave alone work) without a UK visa. This was another special treatment the UK got Your statement "forcing the UK..." isn't quite true. And the "who became public charges and security problems" were the immigrants the UK allowed on it's own volition